crustal magnetism

(magnetic fields independent of a world's core magnetic field)

The term crustal magnetism refers to magnetic fields that
do not stem from the core of a world (planet or moon), but
are from magnetic material on or near the surface, i.e.,
in the planet's crust. Earth has crustal magnetism that
is not merely part of the dipole magnetic field through its core.
In the case of Earth, it is considered to be a remnant
in rocks that include, for example, some iron, from
Earth's earlier magnetic field, which can differ from
Earth's surface geologic activity, including continental drift.

Some worlds, such as the Moon and Mars have
magnetism at the surface but no core magnetic field,
and the crustal magnetism is considered a possible remnant
of an earlier core magnetic field formed by a dynamo.